Plant
Doctor Archive

Potato
blight

I
HARVESTED two buckets full of lovely purple potatoes from six seed
potatoes, but was deeply disappointed to find most of them were
unusable because of a nasty brown blemish inside. Can you please
explain the cause of this and how it might be avoided next season?

FROM
the photo it looks like the problem is caused by one of the blight
diseases that can affect potatoes. Other symptoms you may have noticed
on the aboveground parts of the plant a few weeks before harvest
include brown patches on the leaves and stems and some leaves shrivelling
and dying.

This type of disease
is worse in warm, wet weather and in poorly drained soil, and some
potato varieties are more susceptible to it than others.

The fungal spores can
remain viable in the soil for many years to affect not only future
potato crops but also related plants like tomatoes. So to avoid
the problem next season, plant new, disease-free tubers in a different
part of the garden where no potatoes, tomatoes or their relatives
have been grown for several years. Make sure the soil is free draining
and be careful not to overdo watering. Also, try to choose a spot
exposed to full sun so there's plenty of fresh air flowing around
the plants. Plant the tubers on ridges and keep building the soil
up as the foliage grows to keep the newly forming tubers higher
and drier than the surrounding soil.

If you do see any symptoms
appearing on leaves, you could spray with a fungicide, such as Bravo,
Champion Copper or Fungus & Mildew Spray, according to the label
recommendations. Also, remove any affected leaves and at harvest
collect up all the plant remains and either burn them or dispose
of them in the rubbish to get rid of a possible source of future
infections.

It's also worth remembering
that potatoes prefer slightly acid soil, so don't be tempted to
apply lots of lime. They also need quite high levels of potassium
for good tuber development, so it pays to apply a balanced fertiliser
at planting time.

Weekend
Gardener, Issue 195, 2006, Page 29

Reproduced with permission from Weekend GardenerThe views expressed here are not necessarily those of the RNZIH